1. Field of the Invention
This invention relates to a method of and apparatus for reading out radiation image information recorded in a stimulable phosphor sheet, and more particularly to a method of and apparatus for reading out image information stored in a stimulable phosphor sheet in the form of a radiation energy pattern by stimulating the stimulable phosphor sheet with a two-dimensionally scanning laser beam and detecting light emitted from the phosphor sheet upon stimulation by use of a photo-electric detecting means such as a photomultiplier.
2. Description of the Prior Art
It is known in the art that a stimulable phosphor is able to store radiation source energy of a radiation such as X-ray, alpha ray, beta ray, gamma ray, ultraviolet ray, etc. when it is exposed to the radiation and that it emits the stored energy in the form of light when it is exposed to stimulating rays. Therefore, it is possible to temporarily record a radiation image on a stimulable phosphor in the form of a pattern on the stored radiation energy and read out the recorded image by exposing the stimulable phosphor to stimulating rays to have the stimulable phosphor emit light and photo-electrically detecting the light emitted from the stimulated phosphor.
For instance, as disclosed in U.S. Pat. No. 4,258,264, it is possible to first record an X-ray image of a subject matter such as a human body on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to X-rays transmitting through the subject matter, and then read out the recorded image by two-dimensionally scanning the phosphor sheet with a laser beam to cause the phosphor sheet to emit light according to the recorded image and photo-electrically detecting the emitted light by use of a photodetector. Thus, the radiation image information is converted to a time series image signal.
Since the light emitted from the stimulable phosphor sheet upon stimulation thereof has no directivity and has very low intensity, it is desirable to integrate the output of the photodetector such as a photomultiplier for every short time period unit corresponding to the picture element of the image.
In the above-mentioned system, a laser beam scanning apparatus is employed. In the laser beam scanning apparatus, a mechanical light deflector such as a rotating polygon mirror or a galvanometer mirror is used for deflecting the laser beam in the main-scanning direction and the stimulable phosphor sheet is moved in the sub-scanning direction. Further, as a clock for the reading out operation, clock pulses generated by a quartz oscillator at equal intervals are generally used. However, it is technically very difficult to cause the laser beam scan strictly at a constant speed by use of the mechanical light deflector. Particularly when a reciprocal mechanism such as a galvanometer mirror driven by a saw tooth wave is employed, it is very difficult to make the scanning speed strictly constant. Consequently, in the finally obtained visible image reproduced by use of the image signal output from the photodetector, there appear jitters or unevenness in density which markedly deteriorate the quality of the reproduced image.